The proposed program will study the response of the skeletal muscle to pulse pressure waves of known characteristics (amplitude, dp/dt, frequency and mean values). The perfusion system which includes a pulse wave generator is used to perfuse the isolated gracilis muscle of dogs. The initial experiments have defined the basic vascular response to alterations in amplitude of pulse pressure over a range of mean perfusion pressures. The data suggest that one portion of the microvasculature, probably arterioles, vasoconstricts when pulse pressure is increased and another portion, possibly precapillary sphincters, vasodilates when pulse pressure increases. Subsequent work will attempt to establish the validity of this interpretation by using capillary filtration coefficients (CFC) to reflect the capillary surface area, giving an indication of the open precapillary sphincters. Measurements of oxygen consumption will reflect the metabolic state of the muscle, one of the determinants of the vascular activity. Additional information will be obtained from impedance and phase measurements which reflect reactance (compliance and inertance) as well as resistance.